The present invention relates to a technique useful in the application to assembling of a semiconductor device having a high integrated and high function type semiconductor chip, particularly to a wire bonding process for making an inter-electrode connection.
This kind of technique is described, for example, in "Electronic Material Separate Volume, VLSI Producing/Testing Apparatus Guide Book," published by Kabushiki Kaisha KOGYO CHOSAKAI (Nov. 18, 1987), pp. 123-129.
In the above literature, bonding methods are classified and there are explained recent and future technical trends on wire bonding.
More particularly, in the semiconductor device assembling process there is generally adopted a so-called wire bonding technique wherein an electroconductive wire is stretched for electric conduction between an external terminal (electrode pad) of a semiconductor chip and an inner lead of a lead frame.
As described also in the above literature, in compliance with the demand for increase in memory capacity of a memory element there has been made promotion of high integration and multi-functioning in microcomputer and logic products. At the same time there is now a tendency to narrow the spacing of external terminals on a semiconductor chip and the pitch of the front ends of inner leads. Also, the wire pitch for bonding tends to become narrower. Where wire bonding is performed under such circumstances, bonding defects are apt to occur, such as short-circuiting of wires with each other (wire short-circuit), or short-circuiting of wire with a semiconductor chip or tab (chip short-circuit or tab short-circuit).
Further, when wire is reduced in diameter to cope with the above circumstances, the wire after bonding is no longer strong enough to maintain its loop shape, resulting in permanent strain of the wire, which causes the aforementioned short-circuits.
In view of the above problems there has been proposed a so-called coated wire comprising an electroconductive wire such as, for example, a gold (Au), aluminum (Al) or copper (Cu) wire and an insulating resin coated over the wire.
The following is a brief explanation about a wire bonding method using the above coated wire.
First, a coated wire drawn out from the front end of a bonding tool is subjected to a high-voltage discharge action to melt and remove the coating material at the wire front end and thereabouts, then the wire front end now in an exposed condition is melt-formed into a spherical shape and this spherical portion is pressed to an external terminal of a semiconductor chip under the application thereto of ultrasonic vibration. In this way a first bonding is performed.
Then, while the coated wire is drawn out from the front end of the bonding tool, the other end side of the coated wire is brought into contact with a predetermined position of an inner lead so that the coated wire describes a loop, and ultrasonic vibration is applied again to the bonding tool to destroy and remove the portion of the coating in contact with the surface of the inner lead by virtue of the ultrasonic energy, whereby both exposing and bonding of the wire are effected at a time.